This project is designed to test the hypothesis that serum growth factors are involved in the control of catch-up growth. The investigation will be carried out in rats undergoing recovery from transient experimentally produced growth retardation by fasting, cortisone injections, or propylthiouracil feeding. Measurements will be made of food intake, body weight and tail length at regular intervals during and after treatment. At different points during and after treatment measurements will be made of serum concentration and pituitary content of somatotropin by radioimmunoassay and of serum somatomedin (sulfation factor) by bio-assay (sulfate uptake by porcine or rat rib cartilage). The times selected for sampling also relate to critical events in recovery of normal proportions of body weight to tail length. Cartilage will be collected from the experimental animals for determination of its ability to take up sulfate in the presence of normal rat serum. Metabolic clearance rate and secretory rate of somatotropin will be determined in individual rats in the three catch-up models during the period of treatment, recovery phase, and after recovery phase is completed. Parabiosis will be established between cortisone treated rats and normal litter mate controls in the post treatment period and at the beginning of the resumption of normal growth rate after discontinuing treatment. Skeletal and soft tissue growth will be measured. Depending upon the response somatotropin and somatomedin will be measured in the sera of the parabiosed control. The latter will also be tested with experimental cartilage. Light and electron microscopy of tibial cartilage will be carried out in these animals by a collaborator of the investigator for correlation when appropriate. In other experiments using the same experimental models food efficiency, assimilation, and metabolism will be studied in order to determine the nature of food inefficiency for growth and maintenance which has been observed after cortisone treatment. These data will aid in clarifying the roles of somatotropin, somatomedin, and nutrition in the control of catch-up growth and proportionate growth in the rat.